The present invention relates to a method of producing a molded article. More specifically, it relates to a method of producing a highly accurate molded article such as an optical fiber fixing member, a waveguide, a lens, a microlens array, a prism or a disk substrate from a preform by a molding method.
For producing various optical devices such as a lens, a glass molding method attracts attention as a highly efficient less expensive mass-production method and comes to be used. The advantages of this molding method are highly accurate reproducibility, productivity at a high rate and easiness in operation. For producing the above molded article for optical use, it is general practice to employ a method in which a preform made of a material for optical use is pressed and molded in a mold while it is under heat. In this case, it is very important whether the preform and the mold well match each other, since the molded article as an end product is particularly required to have quality stability.
For this reason, for example, JP-A-5-40694 discloses a method of producing a glass lens, in which a preform is processed into a conical form and the processed preform is set in the central portion of a mold with a positioning tool and press-molded. JP-A-4-108620 discloses a method of producing an optical device, in which a molding material having the form of a column and having at least one edge portion chamfered or spherically processed is press-molded. Further, JP-A-5-301732 discloses a method of producing an optical device, in which a preform having a chamfered portion on a circumferential surface is provided and the preform is press-molded in a state where a clearance between the preform and a mold is as small as possible.
When a thin optical substrate having a fine pattern on its surface, for example, an optical fiber guide block which is used for optically connecting a plurality of optical fibers to a plurality of optical fibers or connecting a plurality of optical fibers to an optical part and which has a V groove pattern for fixing fiber cores on an upper surface and a setting portion for fixing a cladding portion on the setting surface at a level lower than the upper surface is produced by a molding method, it is required to deform a molding material (preform) to a greater extent if the above fine pattern and a step are to be formed at the same time. Further, for forming the V groove pattern, for example, at an accumulated pitch accuracy of within xc2x10.5 xcexcm, it is desirable to use a preform having the form similar to the form of an end product.
The present inventors therefore proposed a method of molding a preform which is provided with a step and which has the form similar to the form of an end product, for securing the transfer accuracy of a fine pattern (JP-A-11-129270 and JP-A-11-133257). When mold members have a gap between them, a molding material is liable to flow through the gap to form burrs, and as a result, the burrs may break away when a molded article is taken out of a mold, to form contaminants or soil the mold and its concave portion. Further, the molded article may be caused to have defects such as cracking and chipping. For preventing the occurrence of burrs, therefore, it was found effective to use a preform which has chamfered portions partly on the front and reverse surfaces thereof and has a step on the front surface side.
When such a preform is prepared by a hot shaping method such as a casting method or a press molding method, the preform imposes almost no problem. However, when it is prepared by a machining method such as a cutting method or a grinding method, procedures according to such a method are complicated and require many processing steps, which causes a problem that a large cost is inevitably required. It has been therefore desired to develop a method of efficiently producing a molded article for optical use from a preform having a simple form with high accuracy and with preventing the occurrence of burrs.
Under the circumstances, it is therefore an object of the present invention to provide a method of producing a molded article such as various products for optical use by press-molding a preform having a simple form, in which the molded article has high accuracy of an outer form, a fine pattern can be transferred with high accuracy and the occurrence of burrs is prevented.
The present inventors have made diligent studies for achieving the above object, and as a result, it has been found that the above object can be achieved by providing a mold comprising a combination of a first mold member having a concave portion having an inner surface formed of a molding surface and a second mold member opposed to the first mold member, and press-molding a preform having a specific form made of a polyhedron that permits fitting of the preform into the concave portion of said first mold member under heat in said mold. On the basis of the above finding, the present invention has been completed.
That is, according to the present invention, there is provided a method of producing a molded article by providing a mold comprising a combination of a first mold member having a concave portion having an inner surface made of a molding surface and a second mold member which is to be opposed to the first mold member and has a molding surface and press-molding a preform under heat in the mold, the method comprising providing, as said preform, a preform (1) which has a polyhedral form that permits fitting thereof into the concave portion of the first mold member and (2) which has opposed first and second surfaces having different sizes (provided that the second surface is positioned in a region of the first surface when viewed as a plan view), a side surface communicating with a circumferential edge portion of said first surface and a circumferential portion surrounded by an edge portion of the side surface and a circumferential edge portion of said second surface, setting the preform in the mold such that the second surface faces said second mold member, and press-molding the preform such that the distance between the molding surface of the first mold member and the molding surface of the second mold member which are opposed to each other can decrease with following a deformation of the preform while the preform is in state where the preform has a temperature equivalent to, or higher than, a temperature at which the preform is moldable.